This invention relates to improved magnesium-based alloys suitable for aerospace applications. The alloys contain lithium and have a crystal structure with two or more phases. In as-cast, wrought or artificially aged forms, the Mg-Li alloys of this invention exhibit improved combinations of properties such as strength, formability and corrosion resistance. The invention further relates to composite structures containing an improved Mg-Li alloy.
It is generally known that magnesium-based alloys weigh less than some light metal counterparts. It is also known that minor additions of lithium improve the weight advantages of magnesium even further. As such, magnesium-lithium offers a viable alternative to aluminum and other light metal alloys for many aerospace applications. Generally, Mg alloys containing around 10% Li are about 45% less dense than aluminum and about 14% less dense than pure magnesium. Mg-Li alloys of this sort also exhibit better ductility and formability properties over more pure magnesium alloys. It is believed that this is due to the dual-phase crystal structure that forms with sufficient lithium addition, said structure exhibiting a hexagonal close packing (hcp) phase with a substantially continuous body-centered cubic (bcc) phase.
In Hesse U.S. Pat. No. 2,622,049, there is shown an age-hardened Mg alloy which includes lithium and at least one metal selected from 4-10% zinc, 4-24% cadmium, 0-12% silver and 4-12% aluminum. Lillie et al U.S. Pat. No. 2,961,359 discloses means for improving the high temperature strength of Mg-Li alloys by heat treating in a preferred atmosphere to convert substantially all lithium to lithium hydride.
Saia U.S. Pat. No. 3,119,689 discloses a Mg-based alloy which includes from 10.5 to 15% lithium, 1 to 3% silver, 1 to 1.5% aluminum, 1 to 1.5% zinc and from 0.1 to 2% silicon. After heat treating for 4 hours at 800.degree. F., water quenching and aging for 24 hours at 225.degree. F., this alloy possesses an ultimate tensile strength of 28 ksi and about 12% elongation.
In Atkinson et al U.S. Pat. No. 4,233,376, a battery anode composition is disclosed which consists of 6-12% lithium, up to 1.5% aluminum and impurities of less than about 0.2%. Japanese Patent Application No. 56/120,293 shows a speaker diaphragm made from a magnesium-based alloy containing 10 to 20% lithium, 0.1 to 1.5% zinc, 0.1 to 1% manganese with trace amounts of Zr, Si, Th and rare earth elements.
In Russia, apparently much research was conducted on magnesium-based alloys. Soviet Patent No. 258,600, for example, discloses a deformable Mg alloy containing 7-10% lithium, 4-6% aluminum, 3-5% cadmium, 0.8-2% zinc and 0.15-0.5% manganese. Later, this cadmium-containing alloy (designated MA-21) was criticized for having low corrosion stability under atmospheric conditions in an article entitled "Electrochemical Behavior of Alloy MA-21 in Aqueous Solutions of Sodium Fluoride", from Zashchita Metallov (Protection of Metals), Vol. 22 (1986).
Soviet Patent No. 455,161 increases the plasticity and "heat resistance" of magnesium-based alloys by adding 7-10% lithium, 0.5-1.5% yttrium, 0.05-0.2% aluminum and 0.05-0.2% manganese thereto. In Soviet Patent No. 485,166, there is claimed a corrosion-resistant Mg alloy which further includes 6-11% lithium, 1-6% aluminum, 3-5% cadmium, 0.5-2% zinc, 0.05-0.5% manganese and 0.05-0.15% rare earth metal.
Soviet Patent No. 559,986 claims another Mg alloy having high levels of lithium, particularly between 12-15%, with 0.5-3% aluminum, 0.05-0.2% manganese, 1.5-5% indium, and 0.005-0.5% chromium. In Soviet Patent No. 569,638, a magnesium-based alloy is claimed to be suitable for rockets, aircraft, space technology, instrument making and other structural materials. For improved foundry and corrosion resistance properties, this alloy contains 10.5-16% lithium, 1-3% zinc, 0.3-3% aluminum, 0.1-0.5% manganese, 0.1-1% scandium, 0.01-0.3% hafnium, 0.001-0.01% boron and at least one other metal selected from 0.05-0.4% neodymium and 0.1-0.3% cerium.